Cancer diseases are among the most important causes of mortality. Radiolabeled drugs, also called radiopharmaceuticals, play an important role in the diagnosis and therapy of cancers. Especially, nuclear medicine is opening new perspectives for diagnostic and functional imaging of tumors, for their characterization (phenotype, proliferation, response to treatment) and that of their environment (vascularization, hypoxia, inflammation, immune response). This characterization of tumors leads to individualized therapeutic strategies. Radiopharmaceuticals are also used in therapy, wherein the vectorization and targeting of radionuclides emitting alpha or beta radiations enables locoregional or systemic therapy.
Radiopharmaceuticals are constituted by two entities: the vector and the radionuclide. Vectors may be peptides, antibodies or organic molecules targeting tumors. Various radionuclides may be used, especially radioactive isotopes of halogens (i.e. radiohalogens), such as for example 125I or 211At. Astatine-211, due to its decay properties (half-life: 7.2 hours; Eα: 5.9-7.5 MeV (100%); multiple X-ray emissions 76-92 keV) is considered as one of the most promising radionuclides for the development of targeted alpha-radionuclide therapy.
The labeling of a vector by a radionuclide to form a radiopharmaceutical may be performed either directly or using a labeled precursor comprising a reactive function able to react with a reactive function of the vector. A commonly used labeled precursor for 211At-labeling of vectors is succinimidyl astatobenzoate (SAB) (scheme 1):

Among methods enabling the introduction of a radiohalogen, especially 211At, halodemetallation reaction of an organometallic compound with an electrophilic species is commonly used (scheme 2):

Due to the high reactivity of the carbon-metal bound, the halodemetallation reaction occurs quickly in mild conditions. The rapidity of the reaction enables radiolabeling compounds with radionuclides having short half-lives while providing high specific activities.
Among organometallic compounds suitable for halodemetallation reaction, organotin derivatives are the most interesting due to the weakness of the carbon-tin bond, making of the tin group a good leaving group. Moreover, tin precursors are easily accessible by conventional synthesis methods from a broad variety of compounds. Especially, commonly used processes of labeling with radiohalogens involve tin(IV) derivatives such as tributyl tin or trimethyl tin (Garg et al., Nucl. Med. Biol., 1995, 22(4), 467-473; Vaidyanathan et al., J. Label. Compd Radiopharm., 2007, 50, 177-182). However, the use of this kind of tin derivatives releases by-products difficult to separate from products of interest leading to low chemical and radiochemical purities and decrease of coupling yields.
Moreover, organotin compounds are known to have an important cellular toxicity. Therefore, any contamination by stannic by-products should be avoided when compounds are dedicated to pharmaceutical or veterinary applications. For these reasons, procedures involving usual tin derivatives are excluded in industrial synthesis of pharmaceutical compounds, despite their synthetic interest.
Solid supported tin reagents have been developed to easily eliminate tin reagents excess from the product of interest and to overcome tin contamination (WO99/18053; Gifford et al., Bioconj. Chem., 2011, 22, 406-412). To the knowledge of the Applicant, the sole example of radiolabeling with 211At using a solid supported organotin reagent was reported by Vaidyanathan et al. for the synthesis of 211At-MABG (meta-[211At]Astatobenzylguanidine) (Vaidyanathan et al., Bioorg. Med. Chem., 2007, 15, 3430-3436):

The synthesis of 211At-MABG was achieved with acceptable yields and good purity (<1 ppm of tin). However, the duration of reaction was quite long and reactivity on solid support was not optimum. Moreover, when using solid supported reagents, it is difficult to automatize the process of synthesis, whereas it is of common practice in radiolabeling processes. Indeed, automatization enables manipulators protection from radiations. Moreover, it accelerates the handling and thus provides higher specific activities and is well-suited to GMP process.
Other attempts have been done recently to overcome tin contamination problems, leading for example to the use of phosphonium grafted organotin (Poupon, et al. Org. Lett. 2007, 9, 3591) and other modified organotin reagents (Olofsson et al. J. Org. Chem. 1999, 64, 4539; Fouquet et al. J. Org. Chem. 1997, 62, 5242; Fouquet et al. J. Chem. Soc. Chem. Comm. 1995, 2387).
There is thus a need for new organotin reagents suitable for halodemetallation reaction to provide radiolabeled compounds with high specific activities and with limited, if any, tin contamination.
In the field of supported reagents, ionic liquids were proposed to replace solid supports. Ionic liquids are onium salts, constituted by the association of an anion and a cation, at least one of which being organic, said onium salts having a melting point below 100° C. The more commonly used ionic liquids have a cation structure centered on nitrogen (tetraalkylammonium, alkylpyridinium, alkylimidazolium), phosphorus (phosphonium), sulfur (sulfonium), 1,4-diazoniabicyclo[2.2.2]octane, sulfethanammonium, prolinium, pyrrolidinium. A large diversity of anions may be used, such as for example halide, acetate, trifluoroacetate, triflate, alkylsulfate, sulfonate, tetrafluoroborate, tetraarylborate, hexafluorophosphate, nitrate, hexafluoroantimonate, prolinate, hydroxide, hydrogen sulfate, tetrachloroferrate, aluminum tetrachloride, perfluorobutylsulfonate, p-toluenesulfonate, formiate, dihydrogen phosphate. The simplest method to exchange the anion of an ionic liquid is ionic metathesis.
As for solid-supported reagents, ionic liquid supported reagents enable simple separation and purification at the end of the reaction, such as for example by filtration on silica, by distillation or by extraction. As for non-supported reagents, ionic liquid supported reagents enable conducting reactions in homogeneous conditions and therefore improve reactivity. Therefore, ionic liquid supported reagents have the advantage to play a dual role of support and solvent. Moreover, in the particular case of a halodemetallation reaction wherein an electrophilic radiohalogen species should be used, the ionic liquid can act as a catalyst for its formation or can enhance its reactivity (Pavlinac et al., Tetrahedron 2009, 65, 5625-5662; Yadav et al., Adv. Synth. Catal. 2004, 346, 77-82).
The Applicant proved the interest of ionic liquid supported organotin reagents for Stille cross coupling reaction, catalytic free radical reduction of alkyl halides and for solvent-free reductive amination (Vitz et al., Green Chem., 2007, 9, 431-433; Louaisil et al., Eur. J. Org. Chem., 2011, 143-149; Pham et al., Chem. Comm., 2009, 6207-6209; Pham et al., Tet. Lett., 2009, 3780-3782). However, to the knowledge of the Applicant, ionic liquid supported organotin reagents have never been used in halodemetallation reaction and even less using radiohalogens.
Considering the potential advantages of ionic liquid supported organotin reagents, the Applicant focused on providing ionic liquid supported organotin reagents suitable for halogenation reaction, especially for the synthesis of “tin free” radiohalogenated compounds. Especially, the Applicant intended providing ionic liquid supported organotin reagents of following formula (I):
wherein X−, n, R1, R2, R3, R4 and R5 are as defined below. Especially, R4 represents an aryl or heteroaryl group, said group having vector properties, or said group being substituted by at least one reactive function able to react with a vector or said group being substituted by at least one substituent having vector properties.
Moreover, it was intended to provide a method of manufacturing of such ionic liquid supported organotin reagents being a reproducible method and a versatile method, adaptable to a large variety of substrates with various reactive functions or vector properties.
A method described in the prior art to prepare ionic liquid supported organotin reagents involves a reaction between the stannylchloride function in the side chain of an ionic liquid with a Grignard reagent (Scheme 4—Louaisil et al., Eur. J. Org. Chem., 2011, 143-149):

In the case of ionic liquid supported organotin reagents of formula (I) wherein R4 is substituted by at least one substituent having vector properties, such bioactive substituents are sensible to degradation. Therefore, harsh Grignard conditions are not suitable for such case.
Another method described in the prior art to prepare ionic liquid supported organotin reagents involves a substitution reaction of an halogen atom in the side chain of a precursor of an ionic liquid, by a stannyllithium derivative (Scheme 5—Vitz et al., Green Chem., 2007, 9, 431-433).

Despite various attempts, above method did not enabled to obtain ionic liquid supported organotin reagents of formula (I) comprising a reactive function. Moreover, the use of very reactive lithium derivatives is not compatible in the case of ionic liquid supported organotin reagents comprising bioactive substituents, which are sensible to degradation.
The Applicant also attempted to adapt method of scheme 5 to prepare ionic liquid supported organotin reagents bearing a reactive function by substituting the halogen atom on an stannylchloride ionic liquid by an aryllithium reactant (Scheme 6).

However, the Applicant showed that the substitution by an aryllithium of a stannylchloride derivative of ionic liquid does not provide an ionic liquid supported organotin reagent comprising a reactive function. Especially, this was evidenced with the reaction reported in scheme 6, wherein none of the expected compound was obtained, while unreactive ionic liquid only was recovered after purification.
Therefore, mere transposition of what was known with ionic liquid as support of organotin reagents is not sufficient to provide ionic liquid supported organotin reagents comprising a reactive function.
Gosmini et al. described a cobalt-catalyzed preparation of non-supported functionalized arylstannanes (Gosmini et Périchon, Org. Biomol. Chem., 2005, 3, 216-217). Especially, the following reaction was described:

Gosmini conditions comprise a first step of activation of zinc dust and cobalt bromide in presence of allylchloride and trifluoroacetic acid in acetonitrile. Then, arylstannane derivatives are obtained in a one-pot reaction from arylbromides or iodide, in presence of tributylstannylchloride, through the passage to the arylzinc derivative.
The mere transposition of above conditions of Gosmini to stannyl chloride ionic liquid did not enable to obtain expected compounds, even less ionic liquid supported organotin reagents comprising a reactive function. Even with some modifications of the conditions, such as varying the number of equivalents or the temperature of reaction, expected compounds have not been isolated.
An important research work was thus conducted to systematically explore all the parameters of the reaction. Especially, it enabled highlighting that very fine zinc dust should be used and carefully activated before use. Besides, the Applicant evidenced that conducting the reaction in presence of dibromoethane enabled to obtain expected compounds in a reproducible manner, even for ionic liquid comprising a reactive function.
Therefore, the present invention provides ionic liquid supported reagents of formula (I) and a reproducible and versatile process for their preparation.
Reagents of formula (I) of the invention may be used in a halodemetallation reaction, leading to halogenated compounds (II), preferably radiohalogenated compounds, as described in scheme 8.

In one embodiment, in compound (II) Y* is preferably a radiohalogen, and compound (II) may react with a biological vector, such as for example an antibody, a peptide or an organic molecule, to provide a radiopharmaceutical (III) useful in nuclear medicine (scheme 9).

In a specific embodiment, compounds (I) of the invention are of formula (I′″a) and react according to scheme 10 to afford intermediate compound (II′″a) bearing a reactive function A able to react with the reactive function B of a vector, leading to radiopharmaceutical of formula (III′″a).

Conditions of radiolabeling with radiohalogen described in the art did not provided expected results. Therefore, an important research work was necessary to determine suitable radiolabeling conditions. The invention thus further relates to a radiolabeling process comprising the reaction of the ionic liquid supported organotin reagent of the invention with a radiohalogen.
The labeled compound (II) may be a radiolabeled vector or can react with a vector, such as an antibody, a peptide or an organic molecule, to provide a radiopharmaceutical (III) useful in nuclear medicine (scheme 9). Reactive function A of the labeled compound (II) and reactive function B of the vector are reactive functions compatible together to form a bound between the labeled compound (II) and the vector, such as for example amine and carboxylic functions leading to an amide bound.
Thanks to the use of the ionic liquid supported reagents of the invention, the purification of the labeled compound (II) may be easily performed in good yields, for example by a filtration on silica gel, distillation or extraction.
Radiolabeling processes are usually performed on automated devices to avoid manipulators irradiation and/or contamination. Moreover, automated devices enable to reduce the time of manufacturing to obtain more important specific activities. Syntheses using ionic liquid supported reagents are performed in homogeneous conditions and with purification methods which present the advantage to be compatible with automated devices. Reactions using non-supported reagents can be automated but require complex, time-consuming and costly systems wherein chromatographic purification unit must be included. Reactions using solid supported reagents require batch process to change the solid substrate.
The Applicant demonstrated that the covalent binding of organotin derivatives on the ionic liquid supported reagents (I) of the invention enables limiting, if any, toxic release of tin when these reagents are used in halodemetallation reactions. Especially, the residual quantity of tin is inferior to 6 ppm, preferably inferior to 3 ppm, in the halogenated compounds obtained using reagents (I) of the invention. Consequently, the tin contamination rate of halogenated products is compatible with pharmaceutical or veterinary applications without further purification as the amount of tin therein is very low. Moreover, as release of tin is avoided, it reduces the environmental impact of the process.
The use of ionic liquid as support instead of solid support also enables to increase the rate of reaction, especially due to a better reactivity in homogeneous medium compared to heterogeneous medium. Increasing the rate of reaction was preponderant more particularly for short half-life radionuclides and leads advantageously to higher specific activities for radiolabeled compounds. Moreover, the use of reagents supported on ionic liquids also opens the possibility to combine effective and fast purifications to innovative automation systems including microfluidic devices.
Therefore, with the ionic liquid supported organotin reagents (I) of the present invention, reactions occur quickly and purification is performed by simple filtration. Radiolabeled compounds with a higher specific activity may thus be obtained. This rapidity of synthesis and purification is all the more important with radionuclides with short half-lives, especially for the 7.2 hours of 211At.
The ionic liquid supported organotin reagents of the invention display the following further advantages:                residual derivatives obtained after halogenation reaction and isolation of compounds (II) may be recycled;        ionic liquid supported organotin reagents (I) and residual derivatives obtained after halogenation reaction and isolation of compounds (II) are odorless and stable at room temperature.        
Therefore, the use of the ionic liquid supported organotin reagents (I) of the invention in the halogenation process of the invention enables the manufacturing of radiolabeled compounds (II) and (III) having a high specific activity, without contamination by tin, for preclinical and/or clinical applications, either in pharmaceutical or veterinary uses.